1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting frames in a wireless communication system including a relay station.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
In order to maximize efficiency of limited radio resources, an effective transmission and reception scheme and utilization methods thereof have been proposed in a broadband wireless communication system. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
Meanwhile, a wireless communication system employing a relay station (RS) has recently been developed. The RS is employed for cell coverage extension and transmission capability improvement. A base station (BS) provides a service to a user equipment (UE) located in a coverage boundary of the BS via the RS, and thus can obtain an effect of extending the cell coverage. In addition, the RS improves signal transmission reliability between the BS and the UE, thereby improving transmission capacity. Even if the UE is located inside the coverage of the BS, the RS may be used when the UE is located in a shadow area.
The RS is generally classified into two types. A first type is a transparent RS which simply relays data, which is transmitted from the BS, to a subordinate RS or the UE while the BS determines all pieces of information required for a relay process. The transparent RS uses the same carrier frequency as that used by a superordinate or subordinate station. A second type is a non-transparent RS which relays data by directly performing operations necessary for the relay process, such as resource allocation, modulation and coding scheme (MCS) level determination, transmit power control, etc. The non-transparent RS may use the same carrier frequency as that used in the superordinate or subordinate station or may use a different frequency.
A centralized scheduling mode is a mode in which a BS determines frequency band allocation of an RS and a relay UE. A distributed scheduling mode is a mode in which the RS cooperates with the BS to determine frequency band allocation for the relay UE. A transparent RS can operate only in the centralized scheduling mode. A non-transparent RS can operate either in the centralized scheduling mode or the distributed scheduling mode.
Examples of a relay scheme used in the RS include an amplify and forward (AF) scheme and a decode and forward (DF) scheme. When using the AF scheme, the RS amplifies data transmitted from the BS and then delivers the data to the UE. When using the DF scheme, the RS decodes data transmitted from the BS to identify a destination station, and encodes the decoded data and then relays the data to the subordinate RS or the UE which is the destination station.
The wireless communication system employing the RS requires a new frame structure different from the conventional frame structure. A frequency band used by the RS to transmit a signal to the BS may be identical to a frequency band used to receive a signal from the relay UE. Alternatively, a frequency band used by the RS to receive a signal from the BS may be identical to a frequency band used to transmit a signal to the relay UE. Due to self interference, it is difficult for the RS to simultaneously transmit and receive signals in the same frequency band. Therefore, a transition gap is required to switch an operation mode between signal transmission and signal reception.
Accordingly, there is a need for a frame structure considering the transition gap and a frame transmission method in a wireless communication system.